FIGS. 1A and 1B show perspective and cross section views, respectively, of a known fuel injection pump, indicated generally as 2, which is suitable for use as a means of supplying pressurised fuel to a fuel injector of an internal combustion engine. The fuel pump 2 includes a generally tubular pump housing 4 having an axially disposed bore 6 within which a pumping plunger 8 is slidable. The pumping plunger 8 has a lower end 10 (in the orientation shown in FIG. 1) that is coupled to a drive arrangement 12 for transmitting reciprocating motion to the plunger 8. The drive arrangement 12 includes a tappet body 14 and an associated cam roller 16 on which a cam member acts, in use (the cam member itself is not shown). A biasing means in the form of a helical spring 17 is received over the plunger 8 such that the spring 17 is disposed between the pump housing 4 and the tappet body 14. An upper end 18 of the biasing spring 17 abuts a spring plate 20 attached to a lower end of the pump housing 4 and a lower end 22 of the spring 17 abuts the tappet body 14, the spring 17 thus serving to bias the plunger 8 downwards in the orientation shown.
As shown in FIG. 1B, an upper end of the pump housing 24 defines a cup-shaped recess 26 into which a lower end of an outlet valve 28 is received. The lower end of the outlet valve 28 closes off the plunger bore 6 and defines a pressurisation chamber 30 between it and the upper end of the plunger 8.
In use, the cam member drives the plunger 8 via the drive arrangement 12 on a pumping stroke during which fuel within the chamber 30 is pressurised. When the pressure of fuel within the pumping chamber 30 reaches a predetermined pressure, the outlet valve 28 opens to permit pressurised fuel to flow through the outlet valve 28. Although not shown in FIGS. 1A and 1B, a fuel conduit may be attached to the outlet valve 28 to convey fuel to a fuel injector, for example.
As the cam member rotates further, the pumping plunger 8 passes a top dead centre position and thus commences a return stroke under the force of the spring 17. During the return stroke, fuel is permitted to fill the pumping chamber 30 through a fill/spill port 32 which is connected to a source of fuel at a relatively low pressure.
In order to vary the delivery volume of the fuel pump 2, the pumping plunger 8 is provided with a control arm 40 which extends radially away from the approximate mid point of the plunger 8. Angular movement of the control arm 40 varies the angular position of the pumping plunger 8.
In use, the control arm 40 engages a fuel delivery rack (not shown) via a control pin 42 that depends downwardly from a radially outer end of the control arm 40. The position of the fuel delivery rack is determined by the engine governor and the rack, in turn, acts on the control arm 40 to cause radial movement of the pumping plunger 8 about its longitudinal axis. The radial position of the pumping plunger 8 determines the point of the pumping stoke that a spill helix 41 (not shown on FIG. 1A) registers with the low pressure spill port 32, thus terminating fuel pressurisation earlier, or later, in the pumping stroke depending on the degree and direction of rotation of the pumping plunger 8. The radial position also controls the start of fuel pressurisation by registration of the upper surface of the pumping plunger 8 with the spill port 32. The variation of the effective stroke between the upper surface of the plunger 8 and the spill helix varies the fuel delivery to the associated engine.
Typically, a plurality of such fuel pumps 2 are installed into the cylinder block of an engine, one per cylinder. In order for the engine to run smoothly, the pumps 2 must be installed with the control arms 40 located in exact positions corresponding to a predetermined delivery setting, hereafter referred to the “reference position”.
Due to production tolerances of the components of the fuel pump 2, each fuel pump 2 provides a given delivery volume with the pumping plunger 8 in a slightly different relative angular position. Thus, each fuel pump 2 is subject to a calibration process during manufacture in which the control arm 40 of each pump 2 is set into the correct position to provide a desired delivery at a given speed defined by a customer, for example an engine manufacturer. Once calibrated, the control arm 40 is locked into its reference position by a locking pin 44 associated with the pump. The locking pin 44 is received within a longitudinally extending bore 46 provided in the pump housing 4 that is approximately parallel to the longitudinal axis of the fuel pump 2.
As can be observed in FIGS. 1A and 1B, the locking pin 44 is supported along substantially its entire length except for its tip 48 that protrudes from the open lower end of the bore 46 to engage a depression or pit (not shown) provided in the control arm 40. It is desirable for the locking pin 44 to be supported close to the spring plate 20 in this way to avoid unwanted movement of the control arm 40 or bending of the locking pin 44 during the process of delivering the fuel pump 2 to a customer. Movement of the control arm 40 would affect the reference position of the control arm, thus negating the pump calibration exercise.
A problem with the above described arrangement is that due to assembly requirements, and the need to support the locking pin 44 along its length, the pump housing 4 is required to be manufactured with a lower portion 50 which is eccentric to an upper portion 52 of the pump housing 4, i.e. axially offset. The process of machining the pump housing 4 to include eccentrically disposed upper and lower portions is complicated and, therefore, expensive. Consequently, it is desirable to provide a fuel injection pump that confers the same advantages and packaging profile as the fuel pump of FIGS. 1A and 1B, but which may be manufactured more readily so as to reduce production effort and overall unit costs.